Heat-sensitive recording materials that make use of the color-forming reaction of a dye precursor with a developer, in which both coloring materials are melted and brought into contact with each other by heating to produce a color image, are well known. Such heat-sensitive recording materials are relatively inexpensive, and recording devices for these materials are compact and easy to maintain. For these reasons, they are used in a broad range of fields, such as recording media for facsimiles, word processors, various computers, and other applications. However, conventionally used heat-sensitive recording materials comprising a developer and solid particles composed of a dye precursor are problematic in that color images thereon decolorize due to heat or humidity.
For the purpose of improving this defect, Patent Documents 1, 2, 3 and 4 propose heat-sensitive recording materials in which a dye precursor is contained in microcapsules. However, when such microcapsules containing an oily liquid prepared by dissolving a dye precursor in an organic solvent are applied to a heat-sensitive recording material, friction and pressure cause increased fogging, resulting in coloration of the background. This problem is effectively avoided by increasing the wall thickness of microcapsules; however this poses a problem that color sensitivity is reduced.
Further, Patent Document 5 proposes a heat-sensitive recording material using composite particles in which a dye precursor is contained in a matrix composed of polyurea or polyurethane resin, the matrix being obtained by dissolving a dye precursor in a solvent comprising a polyvalent isocyanate compound-containing polymerization component, emulsifying and dispersing the solution in a hydrophilic protective colloid-containing aqueous solution, and warming this emulsion. Although the heat-sensitive recording materials using such composite particles containing a dye precursor have a certain level of heat-resistant preservability, heat-sensitive recording materials free from background fogging even in a higher-temperature environment have increasingly been desired.
Moreover, Patent Document 6 proposes a heat-sensitive recording material using composite particles that are produced by emulsifying and dispersing a solution, wherein the solvent is a polyvalent isocyanate compound and the solute is a dye precursor, in a hydrophilic protective colloid-containing aqueous solution, and then accelerating a polymerization reaction of the polyvalent isocyanate compound, wherein a specific amount of a water-soluble aliphatic polyamine compound is added to the protective colloid solution. However, the heat-sensitive recording material using the thus-produced composite particles has been desired to suppress the generation of background fogging when exposed to a high-temperature environment.    Patent Document 1: Japanese Examined Patent Publication No. 1992-4960    Patent Document 2: Japanese Examined Patent Publication No. 1992-37796    Patent Document 3: Japanese Examined Patent Publication No. 1992-37797    Patent Document 4: Japanese Examined Patent Publication No. 1993-63315    Patent Document 5: Japanese Patent No. 3085187    Patent Document 6: Japanese Patent No. 3446553